The field relates to a gas sensor and in particular an electrochemical carbon dioxide sensor for applications in the environmental, medical, agricultural, bio-related and food industries including the food packaging and the brewing and carbonated drinks industry.
Carbon dioxide (CO2) is a colorless, odorless and non-combustible gas and is one of the most important gases on the planet. Plants use CO2, people exhale CO2 and CO2 is one of the most plentiful by-products of the combustion process in devices ranging from furnaces to lawn mowers to coal fired electrical power plants. When present in high concentrations in the air (greater than about 70,000 parts per million (ppm)) it acts primarily as a simple asphyxiant without other physiological effects. In indoor environments, it is primarily produced by human metabolism and is exhaled through the lungs.
Monitoring of carbon dioxide emissions from various natural and industrial sources to the environment facilitates a better understanding of the fate of the carbon dioxide in the global carbon cycle, and in indoor environments, monitoring carbon dioxide levels provides for better quality indoor air through feedback control demand ventilation systems.
Monitoring of carbon dioxide levels in patients, in a hospital or clinical setting, is also important because of the central role of carbon dioxide in physiology. Carbon dioxide is a product of the oxidation of energy sources at the cellular level; it is transported in blood and, for the most part, eliminated through the lungs. Thus it is involved in tissue perfusion and metabolism, systemic circulation, lung perfusion and ventilation. It is expected that changes in those basic functions can be indicated or marked by changes in expired CO2, usually expressed as the end tidal partial pressure of CO2 (petCO2), measured as the plateau section in a capnograph. In addition to end tidal monitoring of carbon dioxide levels in patients, transdermal and sublingual monitoring are alternative methods that provide good correlation with blood carbon dioxide levels.
Furthermore, monitoring of carbon dioxide levels is important in:                A) Agricultural and bio-related process applications: The growth rate and development of plants can be improved by controlling the concentration of carbon dioxide. In greenhouses and mushroom farms, the growth rate and development of mushrooms and plants—from cucumbers to most luxurious roses—can be improved by controlling the concentration of carbon dioxide. This raises the productivity and quality of the crops. Furthermore, measuring and monitoring of dissolved carbon dioxide levels in plant cell culture bioreactors is important for plant physiology research.        B) Food packaging industry: Adding carbon dioxide to food packaging can considerably extend the storage and shelf life of meat, cheese as well as fruits and vegetables. In the meat packaging industry, a high concentration of CO2 in the packaging inhibits bacterial growth and retains the natural color of the meat.        C) Brewing and carbonated drinks industry: Measurement and control of carbon dioxide level is important in these beverage applications.        
In addition to the applications listed above, measurement and control of carbon dioxide levels are important wherever dry ice is produced, handled and used (e.g., food freezing, cold storage, cargo ships and dry-ice production facilities).
There is a large number of carbon dioxide detectors on the market today, designed for environmental, medical and food process monitoring applications. The method of infrared technology is predominantly used in all commercially available detectors. An infrared source at the end of a measurement chamber emits light into a gas chamber, where any carbon dioxide gas present absorbs part of the light at its characteristic wavelength. The absorbance is proportional to the concentration of CO2 in the gas sample.
Systems using the infrared technology are relatively large and expensive and suffer from some limitations when certain situations may affect the reliability of the carbon dioxide measurement. The infrared spectrum of CO2 has some similarities to the spectra for both oxygen and nitrous oxide. High concentrations of either or both oxygen or nitrous oxide (a greenhouse gas) may affect the sensor's reading and, therefore, a correction factor should be incorporated into the calibration of any detector used in such setting. Furthermore, controlling the humidity of the sample gas is important for the accuracy of the infrared measurements. For example, some non-dispersive infrared analyzers for environmental applications use two glass cryotraps: one to dry the ambient air samples and the second to dry the reference gases.
Currently there are two alternatives to the infrared technology. The first is a colorimetric chemical indicator which is a qualitative measurement of carbon dioxide that changes color in the presence of CO2. The second is a Severinghaus-type sensor, which is based on a pH sensor, where carbon dioxide penetrates into the electrolyte of the sensor and changes the pH value, which can be measured by potentiometric, amperometric or other methods. However, Severinghaus type sensors are affected by electromagnetic disturbances because of their high impedance and are difficult to assemble and use. Carbon dioxide measurement is indirect (pH) and an inverse logarithmic function of pCO2 (carbon dioxide partial pressure). The measurement requires maintenance of a delicate film of 0.1M NaHCO3 solution between a thin glass membrane and a CO2-permeable Teflon membrane. There are problems with bubble formation, drying of the electrolyte, and dilution by water vapor and it appears to be necessary to calibrate the sensor frequently.